Yeah,I've never been able to calculate the optimum tube length either but I failed algebra when I told the teacher that pi are round,cornbread are square.
Are you dyno testing your dirt bikes?
Here's another site that has a tube dimension calculator for us mathematical cavemen;
http://www.wallaceracing.com/header_length.php
Are you dyno testing your dirt bikes?
Here's another site that has a tube dimension calculator for us mathematical cavemen;
http://www.wallaceracing.com/header_length.php